1. Field of the Invention
The present invention relates to an elevator apparatus, and particularly relates to an elevator apparatus which is capable of controlling braking force for emergency stop.
2. Prior Art
Generally, in elevators, a rope is wound around a drive sheave of a hoist, and has a car and a counterweight attached to the opposite ends thereof such a well bucket. By rotating the drive sheave the car or cab and the counterweight are vertically moved. The hoist is provided with a braking unit. The car is retarded by an electric brake of the motor of the hoist until the car reaches to a desired story, and then the car is held still by activating the braking unit.
In such elevators, the car is suddenly stopped by actuating the braking unit for emergency stop. To reduce a shock to passengers in the car in emergency stop, a representative attempt has been made in Japanese patent unexamined Publication No. 60-148879, for example, in which an electromagnetic brake unit is used for releasing braking forces of mechanical springs. When an emergency stop command is issued, it has been taken into consideration that braking forces of the springs are reduced by decreasing energizing current of the electromagnetic brake unit.
In the earlier attempt, it is very hard to reduce braking force of the springs by decreasing energizing current of the electromagnetic brake unit. Braking force by the springs rapidly increases since once energizing current is reduced, it rapidly drops. For this reason, the rope of the car is liable to slip over the drive sheave, which is being rapidly decelerated, causing the stop distance of the car to be enlarged. The friction coefficient between the rope and the drive sheave is greatly reduced after slip of the rope is started, so that the amount of slip of the rope becomes excessively large, and passengers in the car therefore have uneasiness.